This invention relates to surgical implants that are designed to replace meniscal tissue and cartilage in a mammalian joint, such as a knee joint, and methods to implant the same. While a knee is the primary joint of concern, the invention applies to other body joints such as the hip, shoulder, elbow, temporomandibular, sternoclavicular, zygapophyseal and wrist. More particularly, this invention relates to a wear resistant hydrogel for such applications.
Compared to the hip, the knee has a much greater dependence on passive soft tissues (menisci, ligaments and the joint capsule) for stability and function. Although the mechanics of the two joints are different, most known hip and knee implants are very similar in design, both consisting of a semi-rigid on rigid (polyethylene on cobalt chrome alloy) bearing surface. In many prosthetic knee implants, function and mobility are impaired because rigid structures are used to replace the natural soft tissues.
Normal anatomical knees joints have two pliable, mobile menisci that function to absorb shock, distribute stress, increase joint congruity, increase contact area, guide arthrokinematics, help lubrication by maintaining a fluid-film bearing surface, and provide proprioceptive input, i.e., nerve impulse via its attachment to the joint capsule. Even under physiologic loading a natural knee with natural menisci will primarily distribute stresses through a fluid film, only 10% of a load is transmitted via a solid on solid contact. Due to the fluid film bearing surface contact wear is greatly reduced. In simple terms the menisci function to reduce joint stresses, decrease wear, and help guide normal kinematics. Without menisci, peak contact stresses in the knee increase by 235% or more and degenerative changes start to progress rapidly. At 0°, 30° and 60° of flexion, natural knees with intact menisci have approximately 6 to 8 times the contact area of typical prosthetic knee implants many of which have a similar geometry to that of a natural knee without menisci.
Typical existing knee replacements lack the functional features normally provided by the menisci and the common polyethylene on metal such as cobalt chrome (CoCr) bearing interface lacks the wear-reducing fluid film bearing surface. By adding a well-designed meniscal substitute, many shortcomings of existing knee replacements can be addressed. In theory, prosthetic menisci could have the same impact on a prosthetic knee as natural menisci do for natural knees.
A prosthetic knee meniscus has at least one and preferably two compliant prosthetic menisci (medial and lateral in the knee) that are attached to the joint capsule and meniscal horns in a similar fashion to the way a natural meniscus is attached to a natural knee. Like a natural meniscus, the meniscal knee implant of the present invention will be able to pivot and glide on a prosthetic tibial plateau. Arthrokinematic constraint comes from the meniscal attachments and will gently guide movements, providing a highly mobile but stable joint. Also through its attachments, the anatomical meniscal-bearing knee will provide proprioceptive input, giving the central nervous system feedback for refined motor control.
In the past, effort mainly has been placed on the development of meniscal replacement. In the attempt to repair or replace torn menisci, allografts, xenografts and autografts have been implanted for over 20 years. Current focus has been on the development of collagen-matrix meniscal implants. However, these implants do not reproduce the mechanical properties of a normal meniscus.
As used herein, all references to “implants” or “implantation” (and all terms such as surgery, surgical, operation, etc.) refer to surgical or arthroscopic implantation of a reinforced or wear resistant hydrogel device, as disclosed herein, into a mammalian body or limb, such as in a human patient. Arthroscopic methods are regarded herein as a subset of surgical methods, and any reference to surgery, surgical, etc., includes arthroscopic methods and devices. The term “minimally invasive” is also used occasionally herein, even though it is imprecise, one should assume that any surgical operation will be done in a manner that is minimally invasive, in view of the needs of the patient and the goals of the surgeon.
Meniscal Tissues in Knees—Each knee joint of a human contains a “medial” meniscus and a “lateral” meniscus. The lateral meniscus is located on the outer side of the leg, directly above the location where the upper end of the fibula bone is coupled to the tibia (“shinbone”). The medial meniscus is located on the inner side of the leg.
Each meniscus (also referred to, especially in older texts, as a “semilunar fibrocartilage”) has a wedged shape, somewhat comparable to a segment from an orange or other citric fruit, but with a substantially larger curvature and “arc.” The thickest region is around the periphery (which can also be called the circumference, the rim, and similar terms). When implanted into a knee, this peripheral rim normally will be anchored to the surrounding wall of a fibrous “capsule” which encloses the knee joint and holds in the synovial fluid, which lubricates the cartilage surfaces in the knee. The two ends of each semi-circular wedge are coupled, via thickened collagen structures called horns to the “spine” protrusions in the center of the tibial plateau.
The inner edge of a meniscus is the thinnest portion of the wedge; this edge can also be called the apex, the margin, and similar terms. It is not anchored; instead, as the person walks or runs, each meniscus in a knee is somewhat free to move, as it is squeezed between the tibial plateau (beneath it) and a femoral runner or condyle (above it). The bottom surface of each meniscus is relatively flat, so it can ride in a relatively stable manner on top of the tibial plateau. The top surface is concave, so it can provide better, more closely conforming support to the rounded edge of the femoral runner. Because of its shape, location and ability to flex and move somewhat as it is pushed, each meniscus helps support and stabilize the outer edge of a femoral runner, as the femoral runner presses, slides and “articulates” against the portion of the tibial plateau beneath it.
However, because all four of the menisci inside a person's knees are in high-stress locations, and are subjected to frequently-repeated combinations of compression and tension (and sometimes abrasion as well, especially in people suffering from arthritis or other forms of cartilage damage), meniscal damage often occurs in the knees of humans, and occasionally other large animals.
It should also be noted that, in humans, meniscal-type tissues also exist in temporomandibular, sternoclavicular, zygapophyseal, and wrist joints.
Various efforts have been made, using prior technology, to repair or replace damaged meniscal tissue. However, because of the complex structures and anchoring involved, and because of the need to create and sustain extremely smooth and constantly wet surfaces on the inner portions of each meniscal wedge, prior methods of replacing or repairing damaged meniscal are not entirely adequate.
Many meniscal implants for the knee address the need for attachment to the surrounding soft tissue but they do not address the need to resurface the femoral and/or the tibial articulating surfaces. An example of this type of implant is described by Kenny U.S. Pat. No. 4,344,193 and Stone U.S. Pat. No. 5,007,934.
A free-floating cobalt chrome meniscal replacement has been designed to cover the tibial bearing surface. Because this implant is rigid and because it is disconnected from the soft tissues it lacks the ability to shock absorb and/or provide proprioceptive input. In fact, because it is approximately 10-20 times more rigid than bone it may actually cause concentrated loading, increased contact stresses, and therefore accelerate degenerative joint changes.
A hydrogel is a network of a hydrophilic polymer(s) in which a large amount of water is present. In general, the water content is at least 20% by weight. In order to keep the hydrogel from being dissolved by the water, the polymer network must be crosslinked either physically or chemically. The water content (and therefore physical size) of hydrogels with either or both types of crosslinks may be sensitive to a variety of environmental conditions depending on the polymer. These environmental conditions include pH, temperature, electric field, and ionic strength and type.
The flexible, pliable gel-like nature of a synthetic hydrogel (when saturated with water) arises mainly from crosslinking attachments between non-parallel fibers in the gel. Depending on the specific polymeric structure that has been chosen, these crosslinking attachments between the long “backbone” chains in a polymer can be formed by covalent bonding, by hydrogen bonding or similar ionic attraction, or by entangling chains that have relatively long and/or “grabby” side-chains.
Regardless of which type of bonding or entangling method is used to bind the backbone chains together to form a hydrogel, the “coupling” points between molecular chains can usually be flexed, rotated, and stretched.
In addition, it should be recognized that the back-bone chains in hydrogel polymers are not straight; instead, because of various aspects of interatomic bonds, they are somewhat kinked, and can be stretched, in an elastic and springy manner, without breaking the bonds.
In a typical hydrogel, the polymeric chains usually take up less than about 10% of the volume; indeed, many hydrogels contain less than 2% polymer volume, while interstitial spaces (i.e., the unoccupied spaces nestled among the three-dimensional network of fibers, which become filled with water when the gel is hydrated) usually make up at least 90 to 95% of the total volume. Accordingly, since the “coupling” point between any two polymeric backbone chains can be rotated and flexed, and since any polymeric backbone molecule can be stretched without breaking it, a supple and resilient gel-like mechanical structure results when a synthetic hydrogel polymer is hydrated.
Physically crosslinked hydrogels are semi-crystalline forms of the polymeric material. The crystalline domains are locations where the polymer chains are neatly packed. The crystalline domains are suspended in the amorphous (i.e., loosely packed, unordered) regions of the polymer, and in order for the crystalline domains to grow they must pull polymer chains from the amorphous regions. As the material becomes more crystalline the equilibrium water content is reduced. The material will continue to become more crystalline until the mobility of the polymer chains in the amorphous regions of the polymer is reduced to the point that they cannot be drawn into the crystalline structure. At this point the polymeric material has reached its equilibrium crystallinity. When using a hydrogel material in an implant, it can be advantageous to ensure that the polymeric material has reached its equilibrium crystallinity prior to being place in vivo so that the material properties and size are stable.
Certain types of ions can help to increase the rate at which polymer chains in the amorphous regions of the material are drawn into the crystalline regions and thus establishing equilibrium crystallinity. The ions that have the greatest effect will depend on the type of polymer. In addition, a greater concentration of ions may increase the rate of crystalline growth. In the case of polyvinyl alcohol hydrogel (PVA), potassium has a greater effect than sodium on the rate of crystallinity (as measured by mass change) when comparing cations. The carbonate ion has a greater effect than chloride when comparing anions. Therefore, potassium carbonate should have a greater effect than sodium chloride on the rate at which a PVA hydrogel will reach its equilibrium crystallinity.
Due to the high water content of hydrogels, there has been interest in using these materials in a variety of medical devices. These devices include those intended for both short (such as a cervical dilator) and long term (such as a non-throbogenic coating for vascular grafts) exposure to the body, and also both load bearing (such as an artificial articular cartilage) and non-load bearing devices (such as contact lenses).
Hydrogels have been used in biomedical applications, such as contact lenses and spinal implants. Among the advantages of hydrogels is that they are as biocompatible as hydrophobic elastomers and metals. This biocompatibility is largely due to the unique characteristics of hydrogels in that they are soft and contain water like the surrounding tissues and have relatively low frictional coefficients with respect to the surrounding tissues. The biocompatibility of hydrogels results in prosthetic nuclei which are more easily tolerated in the body. Furthermore, hydrophobic elastomeric and metallic gels will not permit diffusion of aqueous compositions, and their solutes, therethrough.
An additional advantage of some hydrogels is their good mechanical strength which permits them to withstand the load on the disc and restore the normal space between the vertebral bodies. The spinal nuclei of Bao et al. U.S. Pat. No. 5,047,055 have high mechanical strength and are able to withstand the body loads and assist in the healing of the defective annuli.
Other advantages of the hydrogels, used in the Bao et al. nuclei, are their excellent viscoeleastic properties and shape memory. Hydrogels contain a large amount of water which acts as a plasticizer. Part of the water is available as free water which has more freedom to leave the hydrogel when the hydrogel is partially dehydrated under mechanical pressure. This characteristic of the hydrogels enables them to creep, in the same way as the natural nucleus, under compression, and to withstand cyclic loading for long periods without any significant degradation or loss of their elasticity. This is because water in the hydrogel behaves like a cushion whereby the polymeric network of a hydrogel with a high EWC is less susceptible to damage under mechanical aid.
Another advantage of hydrogels is their permeability to water and water-soluble substances, such as nutrients, metabolites and the like. It is known that body fluid diffusion, under cyclic loading, is the major source of nutrients to the natural disc. If the route of this nutrient diffusion is blocked, e.g., by a water-impermeable nucleus, further deterioration of the disc will ensue.
In addition, the incision area on the annulus can be reduced, thereby helping heal the annulus and prevent the reherniation of the disc. Hydrogels are also useful for drug delivery into the disc due to their capability for controlled release of drugs. Various therapeutic agents, such as growth factors, long term analgesics, antibiotics and anti-inflammatory agents can attach to the prosthetic nucleus and be released in a controllable rate after implantation of the nucleus in the disc.
Furthermore, dimensional integrity can be maintained with hydrogels having a water content of up to about 90%. This dimensional integrity, if the nucleus is properly designed, will aid in distributing the vertebral load to a larger area on the annulus ring and prevent the prosthetic nucleus from bulging and herniating.